The invention relates to artificial reverberation apparatus comprising a first signal combining circuit having a first input coupled to an input terminal, a second signal combining circuit having a first input coupled to the output of the first signal combining circuit via a delay device, a feedback circuit coupling a first point on the signal path between the output of the delay device and the first input of the second signal combining circuit to a second input of the first signal combining circuit, and a transmission circuit coupling a second point on the signal path between the output of the first signal combining circuit and the input of the delay device to a second input of the second signal combining circuit in such manner as to circumvent said delay device. The gain of said transmission circuit is chosen so that the ratio of the gain in the signal path from said second point to the output of the second signal combining circuit via said transmission circuit to the gain in the signal path from said second point to the output of the second signal combining circuit via said delay device is substantially equal and opposite to the gain around the circuit loop including the delay device and the feedback circuit, which gain is less than unity.
An apparatus of the above type is the subject of an article in the Proceedings of the IEEE, August 1977, pages 1108 to 1137, see especially page 1127, FIG. 27. Use of this known apparatus gives a listener the impression that a sound signal derived from its output is accompanied by an echo signal which is delayed with respect to the primary signal by a time corresponding to the delay produced by the delay device. Multiple echoes are often required, i.e. echo signals which correspond to different delays and in such a case a plurality of such apparatuses could be connected in cascade, but this demands the use of a plurality of delay devices each producing the relevant delay. If these delay devices take the form of charge transfer devices, for example bucket brigade or charge coupled devices, a considerable number of storage locations will be required.